CI: Fix YAML syntax

Author: ysangkok

.github/workflows/haskell-actions.yml

@@ -29,7 +29,7 @@ jobs:
       - name: Extract New-Versions git trailer from Renovate
         if: ${{ github.event_name == 'pull_request' }}
         env:
-          GITHUB_HEAD_REF: {{ github.head_ref }}
+          GITHUB_HEAD_REF: "{{ github.head_ref }}"
         run: |
           if [ ! -f cabal.project ]
             then echo 'packages: .' > cabal.project